Did Jack test a prototype of the Screw on the ship in previous drafts? It strikes me as something Graveworthy would never let him do, because of the lack of secrecy. We know the entire ship is full of eyes because of that reporter who knows Graveworthy, and if people are willing to kill Jack they're certainly going to have their own people watching him while he's on that steamship, on top of all the passengers who would probably give information about him for some money. Graveworthy seems way too careful to let this happen here, even if it does move the plot along faster.